The present invention is directed to a waveform generator that can rapidly switch high voltage, direct current on and off in nanosecond speeds to create a pulse or an alternating current.
Rapid switching pulse generators are known. For example, US Patent publication 2015/0318846 by Prager et al. teaches a nanosecond pulser that includes one or more switch circuits including one or more solid state switches, a transformer, and an output electrically coupled with the secondary winding and may output electrical pulses having a peak voltage greater than about 1 kilovolt and a rise time of less than 150 nanoseconds or less than 50 nanoseconds.
Electricity is transmitted from generators to users as high voltage alternating current. High voltage is used to minimize resistive heat loss in the transmission lines. Alternating current is used because it is the most economical form of generation and transmission on large scales, such as power generation for large cities. Use of direct current on such a scale has proven difficult.
One method for converting direct current (DC) to alternating current (AC) comprises using a DC power source connected to switches, whereby the switches are actuated rapidly to alternate the direction of the flow of current, thus creating alternating current.
Current attempts to rapidly cycle switches on and off have been plagued by stray capacitance, also known as parasitic capacitance, the unintended and unwanted capacitance in a circuit. Presently, a device capable of generating high voltage direct current can be built, but when such a device is used to convert DC to AC, the stray capacitance creates long delays, making a practical DC to AC conversion impossible. It only gets worse with scale: as the voltage increases, so does the delay, exacerbating the problem.
One of the oldest methods of creating high voltage in the prior art uses a step up transformer with an inductive flyback circuit. Stray capacitance and inductance within the transformer severely limits the rise and fall rates. This method cannot supply various pulse shapes. The inductive flyback method quickly drops back to zero volts because the transformer cannot sustain such a high voltage. With plasma thrusters it is beneficial if the pulse can fall slowly to avoid negative ionization, rise quickly to produce copious positive ionization, and then a high voltage to accelerate the ions. Other applications need other waveforms. Other methods of high voltage DC power supplies are well known. High voltage pulse generators that use step up transformers and spark gaps are well known.
Another method uses coaxial cables charged to high voltages. This is intended for voltage rise and fall speeds of about one nanosecond and produces square pulses with pulse widths of up to a few nanoseconds. This method cannot supply bias-plus-pulse or other wave shapes.
High voltage transformers have been proposed to address stray capacitance. For example, U.S. Pat. No. 7,417,522 to Sung et al. teaches a high voltage transformer that can be fabricated in a small size and in a cost-effective manner, and designed to minimize stray capacitance generated between components.
Power boosters have been proposed that focus on DC power. For example, US Patent Publication 2014/0268931 by Vogel taches a DC-to-DC boost converter for converting power received from a variable, low voltage DC source, comprising a plurality of interleaved, isolated, full-bridge DC-DC converters arranged in a Delta-Wye configuration and a multi-leg bridge.